Field of the Invention
The present invention relates to an elastic structure for absorbent sanitary products. The invention also relates to a method and an apparatus for producing this elastic structure.
More precisely, the invention relates to an elastic structure including a sleeve comprising two layers of non-elastic flexible material, for example nonwoven fabric, and at least one elastic thread, which extends within said sleeve and is anchored to said layers of non-elastic flexible material at connecting areas spaced apart from each other in the longitudinal direction.
Description of Prior Art
The technique generally used to produce elastic structures for diapers and the like envisages the tensioning of at least one elastic thread in the longitudinal direction within a tubular sleeve of non-elastic flexible material, and fixing the thread to the tubular sleeve at the connecting areas spaced apart in the longitudinal direction, while the elastic thread is in a tensioned state. When the thread tension is released, the tubular sleeve assumes a pleated form, with waves or folds between said connecting portions.
Elastic structures used for the edges of the openings for the legs and the leg cuffs of diapers and similar absorbent products are usually provided with elasticized portions alternating with non-elasticized portions. In the elasticized portions, the elastic threads impart the characteristic pleated forms to the flexible sleeves. The elasticized portions are located in the crotch area of the diaper. The portions of the elastic structures adjacent to the front and rear waistline usually are not elasticized to improve the fit of the article.
A traditional technique to produce elasticized structures with elastic portions alternating with non-elastic portions, in production lines of diapers, envisages the tensioning of an elastic thread in the longitudinal direction and fixing, by means of glue drops, the tensioned elastic thread to a substrate of non-elastic flexible material, for example nonwoven fabric or polyethylene. The application of the glue drops is carried out by controlled dispensing nozzles in phase with the advancing of the substrate and the elastic. The glue dispensing is interrupted to form the non-elastic portions of the structure, so that the elastic thread is not axially connected to the tubular sleeve in the non-elastic portions. In this way, when the individual articles are separated from each other with a cutting operation, the part of the structure that has the elastic thread fixed with the glue takes on the characteristic pleated form and presents characteristics of elasticity, while the part of the structure where the glue dispensing was interrupted remains smooth and non-elastically extensible since the elastic can slide freely inside it in an untensioned state.
The use of glue to anchor the elastic threads to a substrate in production lines of diapers entails considerable problems, including the difficulty of ensuring the dispensing of constant quantities of glue, the possibility of clogging of the glue dispensing apparatuses, the risk of contamination of parts of the system, and the need to carry out welding to close the elastic structures after anchoring the elastic threads.
To overcome the problems arising from the use of glue to anchor the elastic threads of an absorbent structure, anchoring of the tensioned elastic thread within a tubular sleeve by means of welding has been previously proposed. The document U.S. Pat. No. 6,291,039 describes an elastic structure comprising a tubular sleeve and at least one elastic thread, which extends inside the sleeve in a longitudinal direction. The thread is tensioned in the longitudinal direction and is anchored to the sleeve by a plurality of connecting portions spaced apart in the longitudinal direction. Each connecting portion comprises two welds that weld together opposite layers of the tubular sleeve. The two welds of each connecting portion have respective proximal surfaces facing towards the thread, spaced apart by a distance less than the diameter of the untensioned thread. When the longitudinal tensioning ceases, the thread tends to expand radially and remains anchored to the connecting portions by interference. The welds forming the connecting portions of the elastic thread are formed by a thermal or ultrasonic welding device comprising two cooperating wheels, one of which is provided on its periphery with protrusions spaced apart in the circumferential direction and provided with respective notches through which the tensioned thread is passed.
This solution, however, creates problems when elastic structures must be produced with elasticized portions alternating with non-elasticized portions. One solution used envisages the cyclical moving apart of the welding wheels in order to interrupt the welding in the non-elasticized portions. The welding wheels must be moved radially relative to each other with a high frequency, up to 1000 times per minute. A process of intermittent welding of this type generates vibrations that produce a decline in quality of the weld, particularly serious in the case of ultrasonic welding.